Most mutations in genes constituting the RAS–RAF–MEK–ERK signaling pathway are reported in mucinous adenocarcinomas of the appendix (nine genes), followed by appendiceal goblet cell adenocarcinomas and well-differentiated neuroendocrine tumors of the appendix, with six genes each. The gene discussed is MAPK1; the disease is neuroendocrine tumor of the appendix, well differentiated, low or intermediate grade.